Shell Clamping Behaviour in a Limpet 541

Home , Limpet, Patella vulgata

The Journal of Experimental Biology 205, 539–547 (2002) 539 Printed in Great Britain © The Company of Biologists Limited 2002 JEB3898

Shell clamping behaviour in the limpet Cellana tramoserica Gary K. Ellem1,*, John E. Furst2 and Kenneth D. Zimmerman2 1Department of Biological Sciences, University of Newcastle, Callaghan, NSW 2308, Australia and 2School of Science and Technology, Central Coast Campus, University of Newcastle, Brush Road, Ourimbah, NSW 2258, Australia *e-mail: [email protected]

Accepted 5 December 2001

Summary The behaviour of clamping the shell against the predator attack, limpets clamped tightly for several substratum may play an important role in the limpet minutes. In response to lifting forces applied to the shell, adhesion mechanism because friction generated by this limpets clamped at a set proportion of the lifting force, behaviour resists dislodgement by shear forces. This paper even if the lift force was a highly dynamic wave proﬁle. describes the development of an apparatus to analyse This behaviour has implications for numerical models that limpet clamping activity in relation to known forces, attempt to describe limpet adhesion because it shows that including simulated wave activity and predator attack. limpets cannot be represented by a simple mechanical The results show that Cellana tramoserica clamps its shell analogue and that the clamping behaviour must be in a closely regulated manner consistent with an active accounted for if useful predictions are to be drawn. role in the limpet adhesion mechanism. Limpets clamped sharply for several seconds in response to single disturbances such as tapping the shell. In response to Key words: limpet, Cellana tramoserica, shell clamping, behaviour, more continuous disturbance simulating a concerted adhesion, defence, force, wave action.

Introduction It is widely acknowledged that limpets ‘clamp’ or ‘hunker suction provides good resistance to hydrodynamic lift, it down’ when disturbed in an effort to prevent dislodgement provides poor resistance to shear forces. In this paper, we (Cook et al., 1969; McAlister and Fisher, 1968). Until now, propose that limpets clamp their shell against the substratum however, there has been no attempt to quantify the clamping to generate a frictional force that resists horizontal shear. Shell response or to investigate its role in the limpet adherence clamping against the substratum uses the vertical adherence mechanism. This lack of information regarding clamping has strength of the suction mechanism to create a frictional force meant that models of limpet adhesion/shell shape have been that resists the shear force of hydrodynamic drag. Fig. 1 unable to include clamping and, hence, treat limpets as outlines the forces exerted on the limpet while exposed to ﬂuid mechanical structures rather than biological organisms that ﬂow. may be able to respond to applied forces (Denny, 2000). This Smith (1991b) showed that the force with which limpets arguably limits the accuracy of these models, particularly their adhere to the substratum is often greater than the force applied ability to provide insight into the selection of limpet structural to the shell during detachment. Although this result may have characteristics and the metabolic costs of adhesion (Santini et been indicative of shell clamping behaviour, the apparatus used al., 1995). by Smith (1991b) (which was designed to investigate the role Shell clamping brings the lower rim of the conical shell of of suction in the adherence of limpets) was not able to isolate a limpet into direct contact with the substratum. This creates clamping from other mechanisms that may lower the pressure friction between the shell and substratum that provides beneath the foot. A complete analysis of clamping behaviour increased resistance to horizontal shear and prevents requires an ability to differentiate between these mechanisms dislodgement. Most studies that observed clamping involved so that the role of each in adhesion may be understood. the role of clamping as a defence against predators (Branch and The ideal experiment to verify the use of clamping by Cherry, 1985; Thompson et al., 2000), although Denny (1988) limpets in resisting drag would be to measure the forces of lift suggested that clamping has a role in resisting hydrodynamic and drag induced by wave action on a number of limpets while drag. simultaneously measuring the force of shell clamping for the Smith (1992) found that limpets use suction while foraging same limpets. Thus, in an ideal experiment, the force of at high tide. This is a problem for limpets because, although clamping could be directly related to hydrodynamic forces, and 540 G. K. Ellem, J. E. Furst and K. D. Zimmerman

Water velocity responses to wave action suggests that energy efficiency, not resistance to maximum wave forces, may be the primary selective force for limpet shell design. Lift Limpet Materials and methods Friction Drag The method developed for the measurement of shell clamping involved removing limpets from their natural Substratum environment and performing measurements in a laboratory Clamping Clamping under controlled conditions. While this technique had the force force obvious disadvantage of placing the limpets in a foreign Adherence environment that may have confounded their behaviour, it had the essential advantage that accurate measurements could be Fig. 1. A force diagram describing the forces acting during wave performed. Accurate measurement was required because the action. The model assumes that the limpet clamps its shell against the substratum in order to generate friction to resist horizontal shear. results showed that the clamping force was often an order of magnitude lower than the adherence force, and therefore difficult to detect in the absence of accurate measurements. the behavioural mechanics of limpet shell clamping would be known. Limpet collection and maintenance Unfortunately, both these tasks are difficult. It is impossible Cellana tramoserica (Holten 1802) longer than 27 mm were to measure directly the hydrodynamic forces that are placed on collected from exposed, sandstone platforms near Terrigal the limpet without resorting to placing the limpet on some (NSW, Australia). Limpets were removed from the substratum sort of force plate. Another technique may be to predict by placing the flattened tip of a diving knife under the leading hydrodynamic forces on the basis of flow measurements and edge of the shell and twisting the knife slowly. The majority shell shape parameters; however, the highly turbulent flow of of limpets collected in this way showed no sign of injury; the intertidal zone limits the precision with which the actual however, less than 5 % of the limpets, mainly those collected forces may be measured, and the magnitude of these errors is while exposed at low tide, showed signs of damage to the likely to dominate any evidence of behavioural regulation. It underside of the foot or musculature. It is thought that these is also difficult to measure the degree of shell clamping without limpets were using a glue-type adherence mechanism, as inserting something under the shell and, in doing so, changing described Smith (1992), and thus suffered greater injury the flow characteristics of the organism. It is not, therefore, because of their high adherence. Damaged limpets were feasible to measure either the hydrodynamic forces placed on discarded at the collection site. the organism in vivo or the behavioural response of the limpets The limpets were placed in a bucket of freshly collected sea to these forces in vivo. water. The bucket was lined with a netting bag to prevent the We decided to construct an apparatus that removed the limpets adhering to the side of the container. They were then vagaries of force measurement inherent in trying to conduct transported to a constant-temperature laboratory (18 °C), experiments in vivo. The apparatus was able to apply lifting where an air hose was inserted into the water to ensure forces to the limpet shell that could be accurately controlled oxygenation. Limpets remained in the bucket and were used and measured, allowing simulation of wave impact or other within 12 h of collection. force profiles, as required. The apparatus was also able to measure the force of shell clamping directly and independently Development of the apparatus of the pressure beneath the limpet foot and therefore could, We developed an apparatus that was able to measure directly unlike the apparatus of Smith (1991b), specifically identify and the forces acting during adhesion. Newton’s first law states quantify clamping for the first time. that, if the limpet remains adhered to the apparatus and does While a number of previous researchers have developed not undergo acceleration, then the sum of the force acting in apparatus to measure the adherence of intertidal organisms each direction must be zero. Hence, for forces acting in the (Branch and Marsh, 1978; Denny, 2000; Grenon and Walker, vertical direction during adhesion: 1981; Miller, 1974; Smith, 1991b; Thomas, 1948), none has attempted to measure limpet clamping responses or, with the FAdherence = FClamping + FLift , (1) exception of Smith (Smith, 1991b), addressed behavioural where FAdherence is the tensile force placed by the foot on the responses. The results presented here suggest that shell substratum, FClamping is the force with which the covering shell clamping behaviour is an active component of the limpet is clamped against the substratum and FLift is the simulated lift adherence mechanism of resistance to wave action. This has force applied by the apparatus to the limpet shell. FAdherence is implications not only for biomechanical models of adhesion not to be confused with the maximum force of adhesion, or but also in identifying the pressures of natural selection on FAdherence at adhesive failure, used in previous studies. shell design. Recognition that limpets have biologically active The apparatus was designed so that it could accurately Shell clamping behaviour in a limpet 541

Computer- substratum required the construction of a force cell that controlled isolated the tensile attachment of the foot. The force cell Pneumatic regulator (Fig. 2) consisted of a central foot plate surrounded by piston a peripheral shell plate. The limpet was placed on the Laboratory cell such that the foot adhered to the central plate while Force air supply the shell contacted the outer plate. The underside of the application Frame central plate was connected to the outer plate via a load cell and a rigid metal frame. This design allowed the Metal measurement of the forces described in equation 1. The attachment force cell measured the force of adhesion (FAdherence), loop and a separate load cell measured the forces acting on Limpet the shell (FLift and FClamping). Shell plate Electrical component Electronic Clamping load cells measurement A PC-based data-acquisition and control system was developed that used a commercially available data- Foot plate acquisition and control card and associated software (Computer Boards Inc.). The card contained a number Fig. 2. Clamping measurement apparatus. The upper load cell was used to of analogue inputs and outputs and supplied 12-bit measure the simulated lift force applied to the limpet by the piston; the lower analogue-to-digital conversion for a maximum load cell measured the force of pedal adhesion. digitising error of ±0.03 N at a logging frequency of 1 kHz. The relatively high degree of digitisation measure FAdherence and FLift using electronic load cells; hence, accuracy was needed to reduce digital noise in later FClamping could be calculated directly from these two comparative analyses of applied and clamping forces. The measurements using equation 1. high logging frequency was needed to ensure accuracy of measurement when applied and clamping forces were Hardware changing rapidly. The apparatus consisted of a mechanical component and an Load cells were a commercially available S-beam design electrical component. The mechanical component was used to using a full Wheatstone bridge configuration, hermetically apply, direct and measure forces (Fig. 2), while the electrical sealed, and a natural frequency of 11 kHz (Cooper component was used for apparatus control, data acquisition, Instruments). Bridge power, and resultant signal amplification, signal processing, storage and analysis. used a strain gauge amplifier integrated circuit (RS components) configured to provide three different gain settings Mechanical component for different force measurement ranges. The mechanical component of the apparatus fulfilled two The cylinder pressure for the pneumatic piston was main tasks. The first of these was to generate and apply a lifting controlled by an electronic pressure regulator controlled force to the limpet shell; the second was to measure the force directly via an analogue output of the data-acquisition and of adherence of the foot. control card. Power for the regulator came from a 12 V Lift forces were generated using a pneumatic piston that laboratory supply. connected to the limpet shell via a load cell to a hook and a metal loop that had been glued previously to the centre of the Calibration shell. The force exerted by a piston is dependent upon the The system was calibrated using 500 g metal blocks at the cross-sectional area of the piston and the pressure within the beginning and end of each data collection period. Calibration cylinder. The force output is therefore independent of the variation was less than 0.01 % of full-scale deflection during extension of the piston shaft from the cylinder. This allowed a the entire experimental period. Calibration consisted of controlled force to be applied to the limpet in a similar way to measuring the output of the load cell in the absence of load and hydrodynamic lift because the limpet was allowed to select its then repeating the procedure with incrementing numbers of own preferred position of adhesion during the application of metal blocks of known mass attached. The data collection the force. This was different from the force application system was calibrated to give force readings in newtons. apparatus of Grenon and Walker (Grenon et al., 1979; Grenon and Walker, 1981), which raised the limpet shell from the Performance substratum at a set rate, causing extension of the foot and hence A trial was conducted to determine the ability of the applied force in a way that was very different from natural mechanical section of the apparatus to follow controlling phenomena such as wave action in which lift is independent of electronic inputs, specifically in relation the rate of change in shell height. the force (dF/dt) supplied by the pneumatic piston. A model Measurement of the force with which the foot adhered to the limpet (suction cap) and live Cellana tramoserica were 542 G. K. Ellem, J. E. Furst and K. D. Zimmerman

10 000 to the adherence plate. Studies of the re-attachment of the foot showed that the anterior and peripheral sections sealed against ) the substratum early in the re-attachment process, often –1

s resulting in the limpet trapping a bubble of air or water under N 1000 the foot. Forward motion causes this bubble to be ejected from the rear of the foot; however, since motion was restricted in this study by the need to position the foot accurately on the sensor, signiﬁcant bubbles remained beneath the feet of limpets used in the study. The size of the bubbles did not appear to echanical output ( 100 inﬂuence behaviour in any way, but a drop of up to an order m of magnitude was observed in maximum adhesion compared of t d

/ with limpets that were able to re-attach normally. f d A lifting force was applied to the re-attached limpets using 10 one of the two techniques mentioned previously (20 simulated 0.1110 100 1000 wave proﬁles; 38 linearly increasing proﬁles). The lift and adherence forces were measured simultaneously, and the force Rate of change in electronic input (V s–1) due to limpet clamping was calculated by subtracting the lifting Fig. 3. Performance curve for the force application apparatus. The force from the adherence force for each simultaneous mechanical output matched that predicted by the electronic input for measurement. A linear regression was performed to determine force application rates up to 1000 N s–1. The line is theoretical. the response of clamping to lift for each individual. ᭝, Cellana tramoserica; ᭺, suction cap.

Results detached using the apparatus at a number of different Cellana tramoserica placed on the sensor showed a electronic control rates. The output performance of the number of small clamping movements of less than 1 N within mechanical section of the apparatus was then compared with the first 2 min. This was consistent with the limpet attempting that predicted from the electronic controlling input (Fig. 3). to orient properly and attach its foot to the sensor. After Rates of force increase achieved by the apparatus were very this initial re-attachment period, no clamping activity or similar to that predicted by the electronic input up to a rate of attempted movement was observed without the application of force change of 1000 N s–1 (Fig. 3). A value of dF/dt>400 N s–1 a stimulus. was considered sufficient for the experiment since this The relaxed state of the limpets after the initial settling exceeded the rate of change for hydrodynamic forces period was taken as evidence that the relocation process did associated with a limpet undergoing extreme wave action, not stress the limpets. The limpets responded with sudden based on the rate of change for an average limpet of basal area clamping actions to individual stimuli such as tapping the shell 12.5 cm2 undergoing a change in water velocity from 0 to or the apparatus. This clamping rose sharply to a maximum 20 m s–1 in less than 50 ms. force of 2–5 N before decaying back to a relaxed state within A number of different force profiles were used during the 1–2 s. This response was thought to be analogous to the investigation. Simulated wave force profiles were adapted from clamping or ‘hunkering down’ of limpets when disturbed in wave force data collected by Gaylord (1999). Wave profiles the intertidal zone. were used so that the regulation of clamping could be studied Prolonged ‘harassment’ of the limpet (continuous tapping) under conditions that simulated the dynamic force regime of induced a more concerted response in which the force of the exposed intertidal zone. Linearly increasing force profiles clamping increased to approximately 25 N and remained at that of 40 N s–1 were also used so that the lifting force could be level until the harassment stopped. When this stimulus was increased to the point of detachment for the limpet. This stopped, the degree of clamping decayed slowly over the next allowed us to observe the behavioural characteristics of limpet 5 min to less than 5 N, often persisting for up to 15 min at clamping across the entire range of limpet adhesion and 1–2 N. Any repeat of the harassment stimulus during the decay provided a stable stimulus so that clamping could be studied period induced an immediate return to higher levels of at the population level. Force was also applied by hand; this clamping and a similar decay period on cessation of the replicated the linearly increasing force profiles, although with harassment. This behaviour illustrates the clamping behaviour less precision. of Cellana tramoserica when under attack from predators such as crabs, birds or fish or, of course, repeated attempts to remove Determination of the degree of clamping the limpet with a flat-tipped diving knife. Limpets were placed on the force cell so that the foot of the The relaxed state of the limpets in the absence of the above limpet attached to the central foot-plate and the shell rested on stimuli and the responses of the limpets to the different types the peripheral shell-plate (see Fig. 2). Small barriers were of stimuli provided evidence that the limpets were behaving in placed around the limpet to limit movement of the foot relative manner similar to what would be expected in their natural Shell clamping behaviour in a limpet 543

7.5 10 Adherence force FAdherence 8

5 FLift 6

) Lift force N

FClamping (

Force (N) 4 Elastic region 2.5 orce F 2 Force difference

0 0 0 100 200 300 400 500 100 200 300 400 Time (ms) –2 Fig. 4. A typical experiment for the limpet Cellana tramoserica Time (ms) exposed to a simulated wave force. The adherence force (FAdherence) Fig. 5. Force/time curve in the absence of the limpet Cellana of the foot was consistently greater than the lift force (FLift). The tramoserica. The transient force difference seen at the onset of the force difference between the adherence and lift force was generated lift force (circled) was a result of elasticity in the double-sided tape by the action of shell clamping by the limpet (FClamping) (hatched connecting the two force sensors. area). environment. This observation provides a degree of confidence was an order of magnitude higher than the 0.16 N threshold set that the observations collected during later experiments for the detection of clamping. involving the application of lift forces were effective representations of the behaviour of the limpets in response to Clamping in response to simulated wave forces lift in their natural environments. Further analysis was performed on the data presented in Fig. 4 to determine the relationship between the degree of clamping Detection of clamping in response to lift forces and the lift force (Fig. 6). The force of shell clamping was The adherence force for each limpet placed on the force cell found to be proportional to the lift force (y=0.38x−0.09, was greater than the lifting force when exposed to a simulated r2=0.99). The limpet was able to maintain this response despite wave profile (Fig. 4). In each case, the limpet remained the dynamic nature of the lift/time profile applied during the attached to the force cell during the experiment and did not experiment (see Fig. 4). The dynamic range of lift used in the undergo acceleration, implying the existence of a third force experiment was comparable with that observed in natural wave such that the net force on the limpet was zero. conditions. The limpet therefore rapidly increased and The difference between the adherence force and the lift force decreased the tension within the pedal musculature in order to was highly dynamic and closely followed the variation in the follow the wave-like lift profile closely. lift force. Analysis of cross covariance showed there was no Taken together, Figs 4 and 6 showed that limpet shell time lag between adherence and lift forces within the 1 ms clamping behaviour has a role in the adhesion mechanism used detection limit of the apparatus. This raised the possibility that by limpets to resist wave forces. Clamping was observed to be the force difference was a result of inaccurate calibration an active process regulated by the limpet in response to between the load cells rather than evidence of limpet shell changes in the lift force. clamping activity. The experiment was repeated in the absence of the limpet, 3 with the two load cells stuck directly together using double- sided tape. This showed that, in the absence of Cellana tramoserica, the difference between the lift and adherence (N) 2 ng force was zero (Fig. 5). In addition, the apparatus was sensitive i

enough to detect the elasticity in the double-sided tape as it lamp

c 1 stretched during the ﬁrst 25 ms of force application. In the ll e

absence of elasticity, the error between the two load cells was sh 0±0.08 N. We therefore interpreted any sustained difference between the adherence and lift of more than 0.16 N as evidence 0

orce of 2 4 68 of shell clamping activity. F These tests allowed us to discount calibration errors in the Lift force (N) force measurement and conﬁrmed the existence of a third force –1 generated by the limpet clamping its shell against the outer Fig. 6. The force of shell clamping as a function of the lift force for shell plate. This clamping force rose to almost 2.5 N, which the limpet undergoing the simulated wave proﬁle shown in Fig. 4. 544 G. K. Ellem, J. E. Furst and K. D. Zimmerman

1 A 4

y 0.8 ilit

3 b a

rob 0.6 e p 2 v

lati 0.4 u m Shell clamping (N)

1 Cu 0.2

0 0 3 6 9 12 –0.1 0.1 0.3 0.5 Shell clamping/lift 8 B Fig. 8. The cumulative probability distribution of shell clamping as a proportion of the lift force for 38 individuals of the limpet Cellana tramoserica. Each limpet consistently clamped at a given proportion 6 of the lift force. The ﬁgure shows the variation in the proportional response among individuals. The curve is described by P(x)=exp{–[(a–bx)/(a–bc)]1/b}, where a=0.1241345, b=0.2015748 4 and c=0.08828305.

no patterns in this variation were discernible among individuals Shell clamping (N) 2 (Fig. 7A,B). Variation was also observed among limpets in the gradient of the clamping trend in response to lift. Measurements were made of the degree of clamping for 38 0 0 5 10 15 individuals, and the gradient of the clamping trend with respect Lift (N) to lift was calculated. Experience showed that the clamping response of each limpet was unchanged between force Fig. 7. The degree of shell clamping in response to applied lift for Cellana tramoserica exposed to linearly increasing lift forces. applications, providing that the limpet was not detached from Limpet clamping displayed a linear trend (A, y=0.43x−0.16, r2=0.98; the apparatus. However, variation in the gradient of the B, y=0.39x−0.27, r2=0.95) with respect to lift; however, differences clamping response existed among the 38 individuals tested, could be observed between limpets (see A and B) both in the slope of showing a range of clamping responses from –0.08 to +0.5 the trend and in the degree of variation about the trend line. times the applied lift force (see Fig. 8). The degree to which the observed variation in clamping was due to measurement error, rather than to variation in the limpet Clamping in response to linearly increasing tensile loads population, could not be determined. Clearly, negative values Observation of clamping in response to simulated wave for the degree of clamping as a proportion of the applied lift action showed that limpets were able to clamp in an active reported in Fig. 8 would be impossible if the limpets were to manner consistent with the use of clamping as part of their remain attached to the substratum. If the lift force is greater adhesion mechanism. It was impossible, however, to predict than the adherence force of the foot, then immediate the maximum force that could be applied to the limpet without detachment should result. These individuals remained attached detaching it from the sensor. The study of responses to to the apparatus, indicating that, in these cases at least, simulated wave profiles did not, therefore, allow the clamping the apparatus may have significantly underestimated the behaviour of the limpet at the limits of pedal adhesion to be magnitude of pedal adhesion. Checks revealed that the examined. An examination of the response of the limpets at the apparatus was correctly calibrated in each of these cases. limits of adhesion required a new set of force profiles that It appears that the location of attachment of the foot may allowed lift to be applied to the limpets in a uniform and influence the measurement of pedal adhesion. Despite comparable way. attempts to hold the limpet in place during attachment to the The clamping response of Cellana tramoserica was apparatus, the limpet could move its foot within the shell in observed in response to linearly increasing lift profiles. The some cases, so that some of the foot adhered to the peripheral force of shell clamping displayed a linear trend in response to shell plate rather than to the central foot plate. The force the lift force (Fig. 7), and this trend persisted while the limpet sensor was therefore able to measure only the adherence force was attached to the sensor. The force of clamping displayed generated by the section of the foot adhered to the central some variation around the linear fit for each limpet; however, plate and, hence, underestimated the adherence force. In Shell clamping behaviour in a limpet 545 some cases, presumably when individual clamping activity in the limpet adhesion mechanism at all times when a suction- was already low, this underestimation caused the measured type adhesion mechanism is being used by the limpet. adherence to be lower than the applied force. This resulted in No limpets were observed to be using a glue-type adherence the negative value observed for the degree of clamping as a mechanism during the trials (Smith, 1992; Smith et al., 1999), proportion of the applied lift. It was impossible to determine so it is impossible to predict whether clamping behaviour is the degree to which clamping was reduced for each active when limpets are using this adhesion method. Our individual; hence, it was impossible to correct for this error observation that limpets ‘hunkered down’ when disturbed in the data. Therefore, while strong clamping was detected in during collecting at low tide provides anecdotal evidence that limpets during the experiment, the actual degree of clamping the clamping mechanism was still active during glue-type would be marginally higher than the evidence presented here adhesion; however, nothing is known of the behavioural suggests. regulation of clamping in this situation. It therefore seems clear that limpets actively clamped their shells against the substratum as part of their adherence Suction and clamping mechanism. This clamping was regulated and was related to Smith (1991b) performed an experiment that positively the potential threat to adherence faced by the individual. When identified suction as an adhesion mechanism in limpets. In the no threat to adherence was evident, clamping was absent. In process of this experiment, he developed an apparatus that was response to predator-like stimuli, Cellana tramoserica engaged able to measure the pressure under the foot of the limpet in in continuous defensive clamping; in response to wave-like comparison with a vertically applied force. This apparatus was action, a linear response to lift was observed that varied among roughly comparable with the one developed here in that it was individuals. able to measure independently an applied lift force and the force of adhesion of the foot (providing that the area of the attached foot could be accurately determined). The results from Discussion this apparatus showed that the force of adhesion due to suction The experimental data presented here report the first was greater than that required to resist the applied force. quantified evidence of shell clamping by limpets. The actively Indeed, this was the evidence used by Smith (1991b) to suggest regulated nature of the clamping observed suggests a role in suction as a viable adherence mechanism. the overall limpet adhesion mechanism. The possible use The findings of Smith (1991b) and the results presented here of clamping in adhesion by limpets has been suggested support the idea that limpets cannot be approximated by the previously; Denny (1988) suggested that it was a mechanism simple mechanical analogue of a suction cap. A suction cap is involved in countering hydrodynamic drag. This is especially a passive device in which the pressure drop beneath the cap is important for limpets using a suction adherence mechanism a direct result of the detachment force placed on the cap. The because, although the foot is able to generate an adherence force of adherence in this instance is therefore equal to the force several times atmospheric pressure in the vertical applied force. The individuals of Cellana tramoserica direction, it is relatively weak in shear (Smith, 1992). Indeed, observed in this experiment used the muscular tissue of the foot it was poor resistance to shear that Smith (1992) used as a actively to develop pedal adhesion in excess of the force distinguishing feature between limpets using suction adherence applied to them. This process of active adhesion is a conceptual and those using glue-type adhesion for individuals adhered to departure from previous adhesion models that treat limpets as Perspex sheets. mechanical devices and, as such, exclude consideration of Clamping of the shell against the substratum causes friction muscular activity and associated metabolic costs (Denny, between the shell and substratum that resists horizontal motion 2000). (shear). It is therefore not surprising that limpets may use shell There are two possible mechanisms by which limpets could clamping as an integral part of their adhesion strategy since reduce the pressure beneath the foot independently of the this allows the limpet to resist both hydrodynamic lift and drag application of a detachment force. The first has been with a pedal suction attachment that is only able to generate demonstrated in the present experiment. Clamping the shell significant resistance to vertical detachment. The present study against the substratum requires additional adhesive force for shows that Cellana tramoserica use clamping to aid in the maintenance of translational force equilibrium. This places resistance to hydrodynamic shear and predator attack. additional tension on the fluid enclosed by the foot and results Hydrodynamic forces generated by wave action are highly in a reduction in the under-foot pressure. dynamic and may vary from zero to maximal values in as little It is also apparent that limpets remain adhered whilst moving as 50 ms for impinging flows on hard-bodied organisms over the substratum, and this suggests that another attachment (Gaylord, 2000). The present study demonstrated that limpets mechanism also operates by which limpets are able to achieve were able to respond in a regulated manner to rates of change adhesion through suction in the absence of shell clamping. A of force of this magnitude. It was also evident that the muscular model for this mechanism was described by Jones and strength of the limpets was such that the degree of clamping Trueman (Jones, 1973; Jones and Trueman, 1970; Trueman, could be maintained up to the limits of adhesion of the foot. It 1969) and involves the foot acting simultaneously in both is therefore likely that clamping adhesion plays an integral part tension and compression. Sections of the foot contract upwards 546 G. K. Ellem, J. E. Furst and K. D. Zimmerman to produce tension in the fluid enclosed beneath the foot fluid, Clamping and limpet adhesion models creating zones of reduced pressure; these are balanced by Denny (2000) has developed an integrated model of the normal forces from the substratum placing other sections of the limpet adhesion mechanism that includes adhesion, shell shape foot under compression. The limpet is able to move across the and resistance to hydrodynamic forces. This model determined rock surface as these zones of reduced pressure migrate across maximal hydrodynamic forces placed on the foot by wave the foot in pedal waves. Hence, the limpet can maintain suction action on the limpet shell. It then compared these forces with adhesion and translational movement in the absence of both an the empirical ability of limpets to resist these forces applied or clamping-derived force. independently in the directions of lift and horizontal shear, and This proposed non-clamping adhesion mechanism is used this to calculate orthogonal stress indices. The model was analogous to suction adhesion produced by cephalopod suckers then used to determine the optimum value of a shape parameter (Smith, 1991a, 1996). In the case of cephalopods, a central that minimised stress on the foot. spindle is retracted to generate a reduced pressure beneath the Unfortunately, when the optimum value of the shape sucker, while the outer edges of the sucker act under parameter was compared with empirical values, it was found compression against the surface. In the limpet, however, this that a large variation in limpet shell shapes existed, and most process occurs in several sections of the foot simultaneously of these were different from that derived from the adhesion/ and involves a complex interaction between muscular activity shell shape model. In response to this, a number of non- and fluid partitioning within the pedal haemocoel (Jones, 1968, hydrodynamic selective forces were identified that may have 1970, 1973; Jones and Trueman, 1970; Trueman, 1969). influenced shell shape and thus limited the application of the The clamping and non-clamping suction adherence model to real situations (Denny and Blanchette, 2000). techniques are both supported by experimental evidence The present quantification of the limpet clamping (Jones, 1968, 1970, 1973; Smith, 1991b, 1992; Smith et al., mechanism facilitates the development of adhesion/shell shape 1993; Trueman, 1969). The behavioural regulation of these models that involve active suction adhesion and proportional mechanisms with respect to each other has, however, yet to be shell clamping in the resistance to horizontal shear. The results investigated. This regulation is important because limpets must indicate that a consideration of energy expenditure should be maintain adherence across a range of movement requirements added to the list of influences on shell shape (Denny and within a dynamic wave force environment. How they do this Blanchette, 2000). has consequences for energy usage and exposure to detachment Selection pressure due to exposure to maximum wave so should be an important factor in describing limpet forces is limited in its ability to shape limpet shells because physiology. Investigation of this interaction awaits the limpet adherence is so high that non-streamlined shapes may construction of an apparatus that is able to measure under-foot persist in all but the most severe of wave conditions. Once pressure and the force of clamping simultaneously. shells have even a small degree of streamlining, they are able to withstand even the largest waves; hence, the selective Substratum-dependence of shear resistance mechanism becomes inoperative (Denny and Blanchette, Grenon and Walker (1981) examined the resistance of 2000). This is in contrast to the shape of limpet shells, which limpets to horizontal shear on a number of substrata and found have a cross section associated with low drag, and may that resistance was substratum-dependent, in contrast to their include fine-scale adaptations such as ribs that induce findings for resistance to vertically applied forces, which was boundary layer separation for early transition to lower independent of substratum. This is consistent with the role of drag coefficients (Denny, 1989). This suggests that shell clamping behaviour in the resistance to horizontal shear hydrodynamics is an important factor in the selection of shell because shell clamping generates frictional forces that are shape; however, the critical hydrodynamic parameter is substratum-dependent in the direction of shear, but are likely to be something other than the maximum fluid substratum-independent for lift. For surfaces to be equivalent velocity. in resisting lift, they need only have a similar surface energy and, hence, provide equal adherence for pedal mucus in the Thanks go to Mr Barry Lees and Mr Bruce Ellem who event that negative pressures are generated beneath the foot aided in the construction of the apparatus. We would also like (Smith, 1991a, 1996). to thank Dr Brian Gaylord for his then unpublished wave data. Given the complex nature of the rocky substratum, it is Professor Mark Denny and Dr Andrew Smith, together with likely that the coefficient of friction between the limpet shell the two referees, are acknowledged for their comments and and the substratum varies greatly within small spatial scales as feedback, which were most helpful. Lastly, we would like to a result of mechanical interactions between the shell and small thank Dr Paul Dastoor for his enthusiastic encouragement and deformations in the surface. Measurement of the coefficient of constructive criticism friction between the shell and substratum is therefore not a simple task and has not been included in this paper. Evaluation of the frictional environment of limpet shells is important if the References Branch, G. and Cherry, M. (1985). 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